1. Field of the Invention
The present invention relates to a thermal printer which performs printing various kinds of information on recording paper pulled out from roll paper.
2. Description of the Related Art
The thermal printer is a printer having such a structure that a particular recording paper (heat-sensitive paper), which develops color by being applied with heat, is sandwiched between a platen roller and a thermal head, and while the recording paper is being fed through rotation of the platen roller, a printing surface (heat-sensitive surface) of the recording paper is heated by a heating element of the thermal head to be developed color, thereby performing printing. As this type of the thermal printer, for example, there is known a platen-open type thermal printer (for example, refer to Japanese Patent Application Laid-open No. 2000-318260).
This type of the thermal printer has such a structure that the thermal head is provided on a chassis side in which the recording paper wound into a roll is received, and the platen roller is provided on a cover side, which allows an opening portion of the chassis to be openable and closeable. Accordingly, when the cover is opened, the platen roller and the thermal head are largely spaced apart from each other. As a result, there is such merit that a replacement operation of the recording paper may easily be performed.
Here, a detailed description is made of a platen-open type thermal printer with reference to the drawings.
As illustrated in FIG. 16 and FIG. 17, a thermal printer 100 has such a structure that, with respect to a main frame 110 which is fixed onto a chassis (not shown) side, a platen roller 104 which is fixed onto a cover side (not shown) may be detachably assembled.
Specifically, the thermal printer 100 includes: a thermal head 102 including a multiple number of heating elements 101 arrayed in a line; a head support member 103 which supports the thermal head 102, and is rotatably supported with respect to a shaft 109 which is fixed to the main frame 110; the platen roller 104 which sandwiches recording paper P (refer to FIG. 16) between itself and the thermal head 102; lock arms 106 (refer to FIG. 16) which retains a bearing 104a so as to hold the platen roller 104 accommodated in a slit 110a of the main frame 110; and elastic members 107 which urge the head support member 103 so that the thermal head 102 is brought into press-contact with the platen roller 104.
As thus constructed, by using the lock arm 106, the platen roller 104 may easily be mounted to the main frame 110, and by releasing the lock arm 106, the platen roller 104 may easily be removed from the main frame 110.
Further, the head support member 103 is configured to rotate about the shaft 109 so as to be brought into press-contact with the platen roller 104 by being urged by the elastic member 107. Therefore, designing is performed so that, when the platen roller 104 was mounted to the main frame 110, printing may be performed while reliably sandwiching the recording paper P between the platen roller 104 and the thermal head 102.
In order to carry out satisfactory printing onto the recording paper P, as illustrated in FIG. 18, the position of the heating element 101 of the thermal head 102 with respect to the platen roller 104 needs to fall within an allowable range (allowable range L1 of heating element). In a case where the heating element 101 does not exist within the allowable range L1 of the heating element, there is a fear in that heat of the heating element 101 may not sufficiently transmit to the recording paper P, resulting in a lowering of a printing quality, such as not being able to obtain a proper printing density.
Here, the allowable range L1 of the heating element is a range which is obtained by subtracting a predetermined margin M from a nip width L2. The nip width L2 is determined by a contact range of the platen roller 104 and the thermal head 102. A center of the nip width L2, in general, does not match in many cases with a contact reference point N between the platen roller 104 and the thermal head 102 (point in a head surface 102a at which a distance to a center O1 of the platen roller 104 becomes shortest). This is because, due to rotation of the platen roller 104 and depending on a surface shape of the thermal head 102 (head surface 102a), deformation of the platen roller 104 becomes nonuniform between rotation upstream and downstream. Further, the margin M is set so as to absorb an error caused by dimensional fluctuations of parts.
Accordingly, the center of the allowable range L1 of the heating element is generally also offset to the rotation upstream side of the platen roller 104 than the contact reference point N. Note that, the center of the allowable range L1 of the heating element is set as an ideal target point of the heating element 101. In this case, a displacement amount between the target position and the contact reference point N is referred to as an offset amount L3.
Then, in order to carry out the printing with respect to the recording paper P with a satisfactory printing quality, it is necessary that the position of the heating element 101 be set so as to fall within the allowable range L1 of the heating element.
Although as described above the nip width L2 is determined mainly based on the contact range between the platen roller 104 and the thermal head 102, in addition, the nip width L2 may change depending on properties such as a stiffness and thickness of the recording paper P to be used, and respective conditions, such as conveying path of the recording paper P. Accordingly, even the allowable range L1 of the heating element may similarly change depending on the above-mentioned respective conditions.
For example, assuming that the nip width of a thick recording paper P is L2 as illustrated in FIG. 18, in a case of a thin recording paper P which is likely to familiar with an outer peripheral surface of the platen roller 104, the nip width become larger than the width L2. Accordingly, the allowable range L1 of the heating element may naturally change, too.
Like this, the allowable range L1 of the heating element may differ depending on the recording paper P to be used, and hence in order to cope with all the kinds of the recording paper P, it is required to position the heating element 101 within a range at which respective allowable ranges L1 of the heating elements are overlapped. Accordingly, as the kinds of the recording paper P to be used is increased, the allowable range L1 of the heating element which may cope with all the recording paper P suffers a limitation to be shortened. Therefore, there arises a necessity to position the heating element 101 with high precision so as to fall within the range. Therefore, there is required a high positioning precision of the heating element 101 with respect to the platen roller 104.
In order to make it easier to respond to those requirements, it is conceivable to increase a contact force (platen pressure) between the platen roller 104 and the thermal head 102 to enlarge the nip width L2, thereby enlarging the allowable range L1 of the heating element itself.
However, if the contact pressure increases, it leads to an increase of a motor load of a motor for driving the platen roller 104. Particularly, in recent years, along with downsizing of an apparatus, downsizing of the motor for driving the platen roller 104 is also advancing. Accordingly, it is a current state that the motor has no such allowance in its torque that the increase of the contact pressure necessary for securing a sufficient nip width L2 may be increased.
Accordingly, it was hard to enlarge, while achieving the downsizing of the apparatus, simultaneously, the allowable range L1 of the heating element itself.
To this end, conventionally, as means for positioning the heating element 101 with respect to the platen roller 104 with high precision as much as possible, it has been conducted to paste the thermal head 102 with respect to the head support member 103 with precision.
Specifically, as illustrated in FIG. 17, first, there is formed, at an end portion of the head support member 103, an abutting portion 108 which serves as a positioning surface between the thermal head 102 and the head support member 103. Then, when the thermal head 102 is pasted onto a pasting surface of the head support member 103, the pasting is carried out under a state in which an end surface 102b of the thermal head 102 is positioned to the abutting portion 108. With this structure, the thermal head 102 may be pasted onto the head support member 103 with precision using an inexpensive jig, which leads to high precision positioning of the heating element 101 with respect to the platen roller 104.
However, within a tendency of achieving downsizing in recent years, positioning precision of the heating element 101 with respect to the platen roller 104 becomes insufficient by the conventional method.
About this point, detailed description is made as follows.
First, it is thought that the following positioning precisions mainly influence against the positioning precision between the platen roller 104 and the heating element 101 (refer to FIG. 17).
(1) Distance D1 between a center axis O1 of the platen roller 104 and a rotation center O2 of the head support member 103.
(2) Distance D2 between the rotation center O2 of the head support member 103 and a pasting surface of the thermal head 102 in the head support member 103.
(3) Distance D3 from the rotation center O2 of the head support member 103 to the abutting portion 108.
(4) Distance D4 from the end surface 102b of the thermal head 102 to the heating element 101.
(5) Pasting tolerance of the thermal head 102 (distance between the abutting portion 108 and the end surface 102b of the thermal head 102) to the pasting surface of the head support member 103.
Among the above-mentioned conditions, the conditions (3) to (5) depend on the positioning precision when the thermal head 102 is pasted onto the head support member 103. In particular, when the thermal head 102 is pasted to the head support member 103, although it leads to an increase of a facility cost, by directly controlling the position of the heating element 101 using an image recognition device, or the like based on the distance to the rotation center O2, the tolerance may further be reduced.
However, even if the increase of the facility cost is accepted to reduce the tolerances of the conditions (3) to (5), there are large tolerance influences generated in the conditions (1) and (2). As a result, there was a case where it was difficult to position the heating element 101 with respect to the platen roller 104 with a required precision.
In short, the head support member 103 is always urged by the elastic member 107, and is adapted to rotate about the shaft 109 as a center. With this structure, when the platen roller 104 is mounted to the main frame 110 by the lock arm 106, the platen roller 104 is adapted so as to be brought into contact with the thermal head 102 by an appropriate contact pressure. In this case, a mounting position of the platen roller 104 to be mounted to the main frame 110 is determined at its designing stage. Specifically, in the conventional thermal printer 100, the mounting position of the platen roller 104 (namely, above-mentioned distance D1) and the position of the heating element 101 are determined based on the shaft 109 which serves as the rotation center O2 of the head support member 103. The above-mentioned conditions (3) to (5) are all established on the premise described above.
Nevertheless, when the platen roller 104 is mounted to the main frame 110, there was a case where, due to the dimension precision of parts of the main frame 110, a fixing precision of the lock arm 106 with respect to the main frame 110, or the like, the mounting position of the platen roller 104 is slightly displaced from a position determined at its designing stage (distance D1 is changed). Even in such case, as described above, the head support member 103 rotates about the shaft 109 as a center by being urged by the elastic member 107, and hence there is no difference in that the platen roller 104 and the thermal head 102 are brought into contact with each other by an appropriate contact pressure. However, an inclination angle of the head support member 103 varies. In other words, than a state illustrated in FIG. 17, the head support member 103 may tilt forward as illustrated in FIG. 19, or may tilt backward as illustrated in FIG. 20. Besides, in a case where the mounting position of the platen roller 104 is displaced in parallel with the head surface 102a, without the head support member 103 from tilting, the position of the heating element 101 is displaced from the center of the allowable range L1 of the heating element.
In such case, even if the thermal head 102 is positioned accurately with respect to the head support member 103, the heating element 101 can not be positioned accurately with respect to the platen roller 104. As a result, there is a fear in that the heating element 101 may be arranged outside the allowable range L1 of the heating element.
In addition, even in a case where if the platen roller 104 is accurately mounted with respect to the main frame 110, and there is no change in distance D1 described above, in a case where the above-mentioned distance D2 is changed due to the dimension precision of parts of the head support member 103, the tilting angle of the head support member 103 still changes. In other words, in a case where the thickness (D2) of the head support member 103 is small (thin) compared to the state of FIG. 17, as illustrated in FIG. 21, the head support member 103 tilts forward. In a case where the thickness of the head support member 103 is large (thick) compared to the state of FIG. 17, as illustrated in FIG. 22, the head support member 103 tilts backward. Accordingly, similarly, the heating element 101 cannot be positioned accurately with respect to the platen roller 104, and there is a fear that the heating element 101 is arranged outside the allowable range L1 of the heating element.